Blood pressure measurement is very important in antihypertensive therapies. According to the guidelines for antihypertensive therapies of WHO/ISH, the degrees of hypertension are classified in accordance with blood pressure values measured every 5 mmHg, and therapeutic methods suitable for the individual degrees are recommended. Therefore, whether appropriate therapies can be performed depends on the measured blood pressure values. Also, as the aging of population advances in recent years, demands for high accuracy and high reliability of blood pressure measurement are greatly increasing in order to prevent circulatory organ diseases and metabolic syndrome on which hypertension has large effects.
Conventionally, measurement methods used for a non-invasive sphygmomanometer for measuring the blood pressure include winding a cuff around a blood pressure measurement portion, and gradually changing the cuff pressure from a pressure higher than the systolic blood pressure (also called a maximum blood pressure) to a pressure lower than the diastolic blood pressure (also called a minimum blood pressure), a microphone method that measures the blood pressure by detecting the Korotkoff sounds as in an auditory method and an oscillometric method that measures the blood pressure by detecting the change in pulse wave superposed on the internal pressure of an internal air bag of a cuff.
In addition, PLT1 has disclosed a double cuff method that further increases the measurement accuracy of the oscillometric method by setting a pulse wave detection cuff in a position slightly shifted toward the periphery from the center of a blood flow blocking air bag. Also, PLT2 has disclosed a technique that displays a two-dimensional graph having a first axis on which the pressing force of a cuff is a variant and a second axis on which the amplitude of a cuff pulse wave is a variant, in order to determine the reliability of a measured blood pressure value.    PLT1: Japanese Patent Laid-Open No. 2000-79101    PLT2: Japanese Patent Publication No. 2-25610